Aqueous dispersions of polyurethanes or polyurethane ureas have long been known (cf. for example Angewandte Chemie, 82, (1970), pages 53 to 63; DT-OS Nos. 2,314,512 and 2,314,513 and U.S. Pat. Nos. 3,920,598 and 3,905,929) and have a high quality level.
Not least, the fact that many of these dispersions are free from emulsifiers contributes towards this high quality level. They contain chemically incorporated hydrophilic centers which impart self-emulsifiability to the otherwise hydrophobic elastomers. This method of producing self-emulsifiability has two principal advantages over the use of emulsifiers:
(1) LESS HYDROPHILIC CENTERS ARE REQUIRED;
(2) THE EMULSIFIER INCORPORATED IS UNABLE TO MIGRATE FROM SHAPED ARTICLES PRODUCED FROM ELASTOMER DISPERSIONS OF THIS TYPE; AS A RULE, BEHAVIOR OF THIS TYPE HAS A CONSIDERABLE EFFECT UPON THE PROPERTY SPECTRUM.
The first feature, in particular, considerably reduces the sensitivity to water of shaped articles produced from self-emulsified polyurethanes. The hydrophilic centers incorporated into known water-dispersible polyurethanes and polyurethane ureas may represent both salt-like, i.e. ionic groups, and also hydrophilic non-ionic groups.
The "polyurethane ionomers" include both chemically fixed cations, i.e., in particular, polyurethanes containing chemically incorporated ammonium ions, and also chemically fixed anions, i.e., in particular, polyurethanes containing chemically incorporated sulphonate or carboxylate groups. The non-ionic, water-dispersible polyurethanes include, in particular, the polyurethanes and polyurethane ureas containing lateral polyethylene oxide chains according to DT-OS Nos. 2,314,512 and 2,314,513 and U.S. Pat. Nos. 3,920,598 and 3,905,929.
Dispersions of these polyurethanes have different, characteristic property spectra, depending upon the type of hydrophilic center. Thus, polyurethane ionomer dispersions remain stable when heated to temperatures up to their boiling point because the solubility of the salt groups present in them is virtually unaffected by temperature, whereas non-ionic dispersions coagulate when heated to only moderate temperatures (approximately 60.degree. C), because the polyethylene oxide side chains gradually lose their solubility in water at elevated temperature. In contrast to ionomers, however, these dispersions are unaffected by the addition of virtually unlimited quantities of electrolytes and remain stable even after freezing and thawing.
The present invention provides new water-dispersible polyurethanes which, in the form of aqueous dispersions, combine the advantage of excellent resistance to frost and electrolytes with the advantage of very good temperature resistance. It has surprisingly been found that water-dispersible polyurethane elastomers of the type in question may be produced by incorporating into the polyurethane both hydrophilic side chains containing ethylene oxide units and also ionic groups. This is surprising because it was found that mixtures of aqueous dispersions of ionic and non-ionic polyurethanes do not show such a combination of desirable properties. On the contrary, mixtures of this type show above all the disadvantages of the individual constituents.
It was also surprisingly found that, by means of the combined incorporation of ionic and lateral non-ionic hydrophilic groups, the overall concentration of hydrophilic groups may be kept considerably lower than is possible where only ionic or non-ionic groups are incorporated without any adverse effect upon the physical property spectrum of the dispersions.
The particle size of the dispersed particles present in an aqueous polyurethane dispersion is closely related to the concentration of hydrophilic groups in the dispersed polyurethane. In general, the polyurethane dispersion is more finely divided, the higher the content of hydrophilic groups incorporated. For example, in order to produce a dispersion having an average particle diameter T, it is necessary to incorporate into the polyurethane to be dispersed a minimum content of x percent, by weight, --SO.sub.3.sup..crclbar. or a minimum content of y percent, by weight, of lateral polyethylene oxide units. Accordingly, a mixture of equal parts of both dispersions, for an average particle diameter T, would contain at least 0.5 x percent, by weight, --SO.sub.3.sup..crclbar. and 0.5 y percent, by weight, of lateral polyethylene oxide units, based on dispersed polyurethane. However, it was found in accordance with the present invention that, for example, where --SO.sub.3.sup..crclbar. and lateral polyethylene oxide units are simultaneously incorporated into an otherwise corresponding polyurethane, it is possible to produce polyurethane dispersions having a maximum average particle diameter T by incorporating much smaller quantities than 0.5 x percent, by weight, --SO.sub.3.sup..crclbar. and 0.5 y percent, by weight, of lateral polyethylene oxide units.
This has several advantages. Thus, the basically hydrophobic polyurethane has to be chemically modified to a far lesser extent. With the reduction in the number of hydrophilic centers, the tendency which coatings produced from polyurethane dispersions of the type in question have to absorb water and swell is reduced, as is the deterioration in mechanical strength in the moist state which may never be completely avoided in the case of dispersions. This is of considerable advantage, especially in cases where the polyurethane dispersions according to the present invention are used for the production of elastic coatings on textile substrates.